FIG. 12 is a block diagram of a conventional radio-frequency (RF) receiver. An RF signal having a frequency ranging from 55.25 MHz to 801.25 MHz is input to an input terminal 1. A single-tuned filter 2 is implemented by a signal variable capacitance diode and receives the RF signal input to the input terminal 1. The single-tuned filter 2 has a tuning frequency varying within a UHF band (367.25 MHz to 801.25 MHz) in response to a tuning voltage input to a frequency variable port 2a. 
An RF amplifier 3 amplifies a signal of the UHF band output from the single-tuned filter 2. An output of the RF amplifier 3 is connected to a double-tuned filter 4 composed of two variable capacitance diodes and having a tuning frequency varying in response to a tuning voltage supplied to a frequency variable port 4a. 
A signal output from the double-tuned filter 4 is supplied to one input port of a mixer 5. The other input port of the mixer 5 receives a signal output from a local oscillator 6 via a frequency divider 7. The mixer 5 mixes the UHF signal from the double-tuned filter 4 with an oscillation signal from the local oscillator 6 to convert the signal output from the double-tuned filter 4 into an intermediate-frequency signal at 45.75 MHz. An intermediate-frequency filter 8 is connected to an output port of the mixer 5 to attenuate undesired components of a signal outside of its range of 6 MHz. A signal output from the intermediate-frequency filter 8 is then amplified by an intermediate-frequency amplifier and output from an output terminal 9.
The single-tuned filter 2, the RF amplifier 3, the double-tuned filter 4, the mixer 5, and the intermediate-frequency filter 8 constitute an UHF signal receiver section 10.
A VHF signal receiver section 11 receives signals of a VHF band from 55.25 MHz to 361.25 MHz through the input terminal 1, and composed of a single-tuned filter 12, an RF amplifier 13, a double-tuned filter 14, and a mixer 15.
The single tuned filter 12 is composed of a single variable capacitance diode and has a tuning frequency varying in response to a tuning voltage supplied to a frequency variable port 12a. The RF amplifier 13 amplifies a signal at the VHF band output from the single-tuned filter 12.
The double-tuned filter 14 is connected to an output port of the RF amplifier 13 and composed of two variable capacitance diodes and has a tuning frequency varying in response to a tuning voltage supplied to a frequency variable port 14a. The mixer 15 has one input port receiving a signal output from the double-tuned filter 14 and has the other input port receiving a signal output from the local oscillator 6 via a frequency divider 16. The mixer 15 mixes the VHF signal passing through the double-tuned filter 14 with the oscillation signal from the local oscillator 6 to convert the VHF signal from the double-tuned filter 14 into an intermediate-frequency signal at 45.75 MHz. A signal output from the mixer 15 is transmitted to an input port of the intermediate-frequency filter 8.
A tuning section 18 is connected between input ports 17a and 17b of an oscillator 17. The tuning section 18 includes a series assembly 21 including a variable capacitance diode 19 and a capacitor 20 connected in series with each other and an inductor 22 connected in parallel with the series assembly 21.
The output port of the oscillator 17 is connected to an input port of a phase-locked-loop (PLL) circuit 23. The PLL circuit 23 supplies tuning voltages from an output port 23a to the variable capacitance diode 19 in the tuning section 18 and variable capacitance diodes in the single-tuned filter 2, the double-tuned filter 4, the single-tuned filter 12, and the double-tuned filter 14 for controlling the oscillation frequency of the local oscillator 6 and the tuning frequencies of the single-tuned filter 2, the double-tuned filter 4, the single-tuned filter 12, and the double-tuned filter 14.
In the conventional receiver, the mixers 5 and 15 output intermediate-frequency signals at 45.75 MHz. This requires frequencies of signals passing through the single-tuned filters 2 and 12 and the double-tuned filters 4 and 14 to be separated by the range of the intermediate-frequency (45.75 MHz) from the frequencies of the signals output from the frequency dividers 7 and 16.
Such conventional receiver receives a wide frequency range from the VHF band to the UHF band with the single local oscillator 6. It is hence not easy to separate the frequencies of signals passing through the single-tuned filters 2 and 12 and the double-tuned filters 4 and 14 by the range of the intermediate frequency from the frequency of the signals output from the frequency dividers 7 and 16. Accordingly, the passing frequencies of the tuned filters may shift from a receiving channel, hence reducing attenuation of any interference signal. As a result, an interference signal may be received directly by the mixers 5 and 15, hence causing image interruption.
Conventional RF receivers similar to the receiver explained above are disclosed in Japanese Patent Laid-Open Publication Nos.2000-295539, 2002-118795, and 1-265688.